(1) Field of the Invention
This invention relates to a method for detecting brazing defect in plate-fin type heat exchangers with aluminum-brazed plates and fins.
(2) Description of the Prior Art
Plate-fin type heat exchangers include the so-called core structure which has wavy fin member alternately between a number of parallelly spaced plate members of aluminum, stainless steel or the like, the contacting joint portions of the fin and plate members being brazed to each other by immersing the core assembly in a hot salt bath. Therefore, in the production of a large-sized plate-fin type heat exchanger, for example, there sometimes occurs a problem in that the brazing of the joint portions of the wavy fins is obstructed over certain areas by air which remains in the core assembly even after immersion in the salt bath.
Such brazing defects in the heat exchanger give rise to a serious problem in that the heat exchanger is ruptured from the defective portions in the subsequent pressure endurance test or during use under high pressure conditions. In this connection, it has not been contemplated or proposed to provide a simple and reliable mechanism for detecting the unbrazed portions, in view of the complicated construction of the plate-fin type heat exchanger. With ordinary steel materials such as steel plates, steel pipes and steel rods, ultrasonic flaw detectors operating with ultrasonic waves of frequencies over several thousands KHz are widely used for internal flaw detection. For detecting internal flaws in such solid materials, the ultrasonic wave to be applied is required to have high directionability and therefore normally is in a high frequency range of 2 M to 5 M. However, in a case where the ultrasonic wave of the high frequency range is applied to a plate-fin type heat exchanger, a marked attenuation of the ultrasonic wave occurs due to its reflections in the cavities of the fluid or gas passages formed in the heat exchanger and as a result such fails to detect the brazing defect. Therefore, the conventional ultrasonic flaw detectors have been unsuitable for or incapable of detecting brazing failures in the plate-fin type heat exchangers containing the fluid passages defined by the plate and fin members.